Adhesives have been used for various industrial purposes. Two major application processes are sprayable and fine-line application of hot melt adhesives. Of particular interest in such application methods are the viscosity of the hot melt adhesive at coating or spraying temperatures and cohesive strength of the adhesive itself. Other important properties include tack, adhesion to substrate and aging.
As base polymers of hot-melt adhesives, various polymers have been used such as polymers such as ethylene-vinyl acetate copolymers, block copolymers of conjugated dienes and vinyl aromatic hydrocarbons, ethylene-.alpha.-olefin copolymers, and polyester resins. Block copolymers are particularly preferred as adhesives because of possessing a relatively favorable balance between adhesive properties and creep resistance. Furthermore, butadiene-styrene block polymers are especially preferred as adhesives due to relatively low cost, excellent balancing of properties and good aging.
U.S. Pat. No. 4,792,584 (Shiraki et al.) discloses an adhesive composition containing a block copolymer containing at least one polymer block composed mainly of a vinyl aromatic hydrocarbon and at least one polymer block composed mainly of butadiene. The block copolymer is characterized by the fact that the vinyl aromatic hydrocarbon content [S] as measured by weight percent of the total block copolymer is 10 to 30% by weight of the block copolymer and the vinyl content [V] of the butadiene monomer contributed units in the block copolymer is 20 to 50%. To prepare an acceptable adhesive, the vinyl aromatic hydrocarbon content [S] and the vinyl content of the butadiene portion [V] must satisfy the following formula: EQU 40.ltoreq.[S]+[V].ltoreq.70.
According to this disclosure if the total vinyl aromatic hydrocarbon content and the vinyl content of the butadiene portion is outside the above range, the initial tack of the resultant adhesive is insufficient.
Applicants have discovered, contrary to the teachings of U.S. Pat. No. 4,792,584, that a block copolymer of high styrene and high vinyl content in the butadiene portion exhibits an excellent combination of low melt viscosity and high adhesive tensile while maintaining other adhesive properties. Block polymers containing styrene contents over 30% and a vinyl content in the butadiene portion which exceeds 35% are particularly preferred. These results are in complete opposition to the teachings of U.S. Pat. No. 4,792,584.
It is therefore an object of the present invention to provide block copolymers exhibiting excellent adhesive tensile strength and low melt viscosity while maintaining other desirable physical and adhesive properties. It is a further object of the present invention to provide adhesive compositions containing block copolymers having high adhesive tensile strength and low melt viscosity.